Each i.MX 6ULL System-On-Module (SOM) comes pre-loaded with U-Boot. U-Boot runs as the primary firmware from the on-module Flash on each power-on / reset.

U-Boot is probably the most popular firmware monitor for Linux. It is developed and maintained by DENX Software Engineering (www.denx.de). If you need detailed information on any aspects of U-Boot operation, DENX publishes extensive U-Boot user documentation at their web site.

On the i.MX 6ULL SOM, U-Boot image is stored in the NAND Flash device. On a power-on / reset, U-Boot is relocated to the SDRAM and runs from there. As soon as the module is powered on or reset, the i.MX6 proceeds to boot the U-Boot firmware printing the following output to the serial console:

U-Boot 2016.11-imx6ull-2.4.0 (Jul 06 2017 - 15:59:15 +0300)

CPU:  Freescale i.MX6ULL rev1.0 at 528MHz
CPU:  Commercial temperature grade (0C to 95C) at 43C
Reset cause: WDOG
Model: Emcraft i.MX6 ULL SOM Board
DRAM:  256 MiB
NAND:  256 MiB
MMC:   FSL_SDHC: 0
In:    serial@02020000
Out:   serial@02020000
Err:   serial@02020000
Net:   FEC [PRIME]
Hit any key to stop autoboot: 0
IMX6ULL-SOM U-Boot >

If you hit any key on the serial console before the number of seconds defined by the U-Boot bootdelay variable has elapsed, you will enter the U-Boot interactive command monitor. From the command monitor you can run U-Boot commands to examine memory, load an image from Ethernet, boot Linux from a loaded image or perform any other action supported by U-Boot.

Use the help command to print out the list of the enabled U-Boot commands:

IMX6ULL-SOM U-Boot > help
? - alias for 'help'
...
IMX6ULL-SOM U-Boot >

As mentioned above, the DENX website publishes detailed documentation describing use of various U-Boot commands.